If we're talking about the same comma, it wasn't necessary. But no, American style requires that you put periods and commas inside of a right quotation mark. That's NOT true with other punctuation; exclamation points, question marks, colons and semicolons.
It's wacky, but that's the way it goes.
I'm getting lost in this conversation. d8-) IIRC, I first thought there was another error, in which you missed placing a comma before a coordinating conjunction. But then I realized it was a subordinating conjunction, which doesn't require one.
I forget. If you want me to go back and look it up, I'll do so. Having raised this silly piece of pedantry I feel responsible, if you care, to get it right.
"Logical" just refers to British style. That's the term you'll sometimes see in literature related to the subject. "Strict" is a term I'm using to refer to the British style of using single quotes first, and then double quotes inside of them. That's the opposite of American style. You'll see this "strict" style in _The Economist_, for example.
In American style, the only uses of single quotes by themselves are literary -- primarily in poetry. You'll see these uses explained in the Modern Language Association (MLA) stylebook, and, IIRC, in the Chicago stylebook.
Right. I'm just talking about publishing style. Electronic communication demands some modifications.
Semicolons are a (re)invention of a 17th-century typesetter, who wanted a pause longer than a comma but shorter than a colon. It had fallen into disuse until he revived it.
Two independent clauses, or as you say, two sentences, are rarely punctuated in speech with a pause as long as a semicolon. It sounds a bit melodramatic. Thus, there are some well-regarded writers who refuse to use it. Kurt Vonnegut had a lifelong, one-man crusade against them.
You'll see such clauses punctuated with commas from time to time, and the pause feels right to me. If punctuation is intended to replicate the pauses and emphases in speech (most linguists agree that it is), the comma is closer to the way we actually say such sentences.